Perimeter security system and perimeter monitoring method

ABSTRACT

A perimeter security system is disclosed which includes a first cable ( 40 ) and a second cable ( 60 ) buried beneath the ground in a zig-zag pattern. The first cable ( 40 ) has a first fibre ( 44 ) and a further fibre ( 42 ). Second cable ( 60 ) has a second fibre ( 62 ). The first and second fibres ( 44 ) and ( 62 ) are connected by a coupler ( 52 ) at one end so that light can be launched into the first and second fibres ( 44 ) and ( 62 ) to propagate in one direction. The further fibre ( 42 ) is connected to a coupler ( 70 ) which also connects to the other end of the first and second fibres ( 44 ) and ( 62 ) so light can be launched into the fibres from the other end and travel in the opposite direction. Detectors ( 80 ) and ( 82 ) are provided for detecting an interference pattern produced by interference of the propagating light signals so that if a person attempts to breach the barrier by walking across the ground beneath which the cables are buried, the cables are moved to change the nature of the propagating light to in turn change the interference pattern to provide an indication of the intrusion. The location of the intrusion can also be determined by the time difference between receipt of the altered interference pattern propagating in the first direction, compared to that propagating in the opposition direction.

FIELD OF THE INVENTION

This invention relates to a perimeter security system and to a method ofmonitoring a perimeter.

Optical devices are commonly used in industry and science in order totransmit data from one place to another. Photronics technology hasrevolutionised the communications and sensor fields due to the rapiddevelopment of optical and opto-electronic devices.

ART BACKGROUND

Our International application PCT/AU99/01028 discloses a system in whichthe location of a disturbance to a fibre and, in particular to acommunication fibre can be determined. This system utilisingcounter-propagating optical signals so that light signals propagate inboth directions along a waveguide. Any attempt to disturb the waveguidewill cause a change in the counter-propagating signals and that changecan be detected by detectors so that a time difference between receiptof the modified counter-propagating optical signals enables the locationof the disturbance to be determined.

Our International application number PCT/AU00/00382 discloses a systemin which both a sensing signal and a communication signal can belaunched into a single waveguide and transmitted along the waveguidewith minimal losses to both the sensing signal and the communicationsignal.

Our International application number PCT/AU00/01332 discloses a methodand system in which perimeter barrier elements such as fence sectionsare spring mounted for limited movement. An optical fibre is connectedto the elements so that any attempt to break in or tamper with the fencecauses the element to move against the bias of the spring which in turnmoves the optic fibre so that a change in a parameter of the lighttravelling through the fibre can be detected in order to provide anindication of the intrusion or tampering.

The contents of the above four International applications areincorporated into this application by this reference.

The perimeter barrier technique disclosed in the abovementionedInternational application provides an extremely efficient monitoringsystem and method for perimeter barriers which include fences or otherphysical elements which are intended to provide a barrier againstingress of individuals. Since the above invention operates by springmounted fence elements having a fibre in proximity to the fence elementso that movement of the element moves the fibre, it is necessary that,in the earlier invention, the perimeter barrier be formed by a physicalstructure to which the fibre is attached. The present invention relatesto a perimeter barrier system in which there is no physical barrierrequired in order to operate the detecting system and which is thereforesuitable for location in the ground to provide security to a perimeterof a required area.

SUMMARY OF THE INVENTION

The invention, in a first aspect, may be said to reside in a perimetersecurity system including;

-   -   at least a first waveguide buried below ground level and        extending along a perimeter which defines an area to be        monitored;    -   means for launching light into the first waveguide; and    -   a detector for detecting light which has propagated through the        waveguide so as to detect a change in a parameter of the light        propagating through the waveguide due to an intrusion across the        ground beneath which the waveguide is buried and for providing        an indication of that intrusion.

Preferably at least a second waveguide is also provided, and the meansfor launching the light, launches the light into both the first andsecond waveguides;

-   -   coupling means for coupling the first and second waveguides        together so that light propagating through the first and second        waveguides is caused to interfere to create an interference        pattern; and    -   wherein the detector detects the interference pattern and upon        an intrusion a parameter of light passing through one of the        waveguides is altered with respect to the same parameter of the        light passing through the other of the waveguides, to thereby        change the interference pattern detected by the detector to        provide an indication of the intrusion.

Preferably the first and second waveguides are provided in at least onecable.

Most preferably the first and second waveguides are provided in separatecables and the separate cables are buried beneath ground level inzig-zag spaced apart relationship with respect to one another to definea perimeter region having a substantial width which will be traversed bya person intruding into the area.

Preferably the substantial width is a width such that a persontravelling in normal walking or running motion will not step over thewidth of the region.

Most preferably the width of the region is between one and two meters.

In the most preferred embodiment of the invention counter-propagatinglight signals are launched into each of the waveguides so that thelocation of an intrusion can be detected by the time difference betweendetection of the changed interference pattern propagating in onedirection and to the changed interference pattern propagating in theopposite direction.

Preferably a first of the said cables contains said at least onewaveguide and a second said cable contains said second waveguide;

-   -   a further waveguide being contained within the first cable;    -   first coupling means at one end of the said first, second and        further waveguides for coupling the waveguides so that light        launched into the said other waveguide is able to propagate        through the other waveguide and then into the said first and        said second waveguides to propagate in a first direction through        the said first and second waveguides;    -   second coupling means at the other end of said first and said        second waveguides so that the light propagating in the said        first direction through said first and second waveguides is able        to coherently recombine and interfere at the second coupling        means; and    -   light also being able to be launched through said second        coupling means and into said first and second waveguides to        travel in a direction opposite said first direction and        coherently recombine at the first coupling means so the light        travelling in the opposite direction is able to interfere and        then propagate through the said further waveguide.

Preferably the detector is coupled to the further waveguide and to thesecond coupling means for detecting the counter propagating lightsignals after interference of those signals so that any disturbance ofthe first waveguide and/or said second waveguide will change a parameterof the light propagating through the first and/or second waveguides tothereby change the interference patterns detected by the detector tocause the detector to provide an indication of the intrusion.

Preferably the location of the intrusion can be determined by the timedifference between receipt of the modified counter-propagating signaltravelling in the first direction compared to the receipt of themodified propagating signal travelling in the opposite direction.

Preferably the detector comprises a first detector and a seconddetector, the first detector and second detector being synchronised andthe first detector detecting the counter-propagating signal travellingin the first direction and the second detector detecting thecounter-propagating signal travelling in the opposite direction.

Preferably the means for launching light into the waveguides comprises alight source coupled to a third coupling means having first and secondoutput arms, the first output arm being coupled to an input arm of afourth coupling means and the other output arm being coupled to an armof a fifth coupling means, an arm of the fourth coupling means beingcoupled to the further waveguide for launching light into the furtherwaveguide, and an arm of the fifth coupling means being coupled to anarm of the second coupling means for launching light into the secondcoupling means.

Preferably the first detector is coupled to an output arm of the fourthcoupling means and the second detector is connected to an output arm ofthe fifth coupling means.

The invention also provides a method of monitoring a perimeter,including;

-   -   providing a first waveguide below ground level along the        perimeter to be monitored;    -   causing a light signal to propagate through the waveguide; and    -   detecting a change in parameter of the light signal to indicate        an intrusion across the perimeter.

Preferably a second waveguide is provided and the light signal islaunched into the first and second waveguides;

-   -   the method including causing the light signal in the first        waveguide and the second waveguide to combine and interfere; and    -   the detecting step comprising detecting the interference pattern        so that a change in interference pattern indicates in intrusion        across the perimeter.

Preferably the method further includes;

-   -   causing counter-propagating light signals to propagate through        the first and second waveguides, detecting modified        counter-propagating signals caused by a change in parameter of        the signals due to an intrusion across the perimeter and        determining the location of the intrusion by measuring the time        difference between receipt of a modified counter-propagating        signal travelling in a first direction compared to receipt of a        modified counter-propagating signal travelling in the opposite        direction.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will be described, by way ofexample, with reference to the accompanying drawings in which;

FIG. 1 is the schematic view showing the layout of the system accordingto one embodiment of the invention;

FIG. 2 is a view showing more detail of the actual perimeter formed bythe system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1 a transmitting and detecting section 5 is shownwhich includes a light source 10 such as a pigtailed laser diode whichlaunches light into first arm 12 of coupler 14. The coupler 14 hasoutput arms 16 and 18 connected to input arm 20 of a coupler 22 andinput arm 24 of a coupler 25. Coupler 14 has an arm 15 which is not usedand couplers 22 and 25 have arms 17 and 19 which are also not used. Arms26 and 28 of the couplers 22 and 25 are connected to connectors 30 and32 by fibres 31 and 33.

The detecting section of the system shown in FIG. 1 comprises a firstcable 40 and a second cable 60. The first cable 40 has two waveguides 42and 44 in the form of optical fibres and the second cable 60 has awaveguide 62. In practice, optical fibre cables generally include atleast two fibres and more commonly at least four or six fibres. For thepurposes of the preferred embodiment of the present invention if thecables 40 and 60 include more optical fibres then, in the case of thecable 40, only two of the fibres need be used and in the case of thecable 60 only one of the fibres is used. The connector 30 connectsdirectly to fibre 42 of the cable 40.

The connector 32 is connected to arm 50 of optical coupler 52. Theoptical coupler 52 has arms 54 and 56 which are connected to the fibre44 and the fibre 62 respectively. Arm 53 of the coupler 52 is not used.

The fibres 42, 44, and 62 pass all the way through the cables 40 and 60respectively and the cables 40 and 60 may have a considerable length ofmany kilometers.

The fibre 42 which exits the cable 40 is connected to arm 71 of coupler70 and the fibres 44 and 62 which exit the cables 40 and 60 respectivelyare connected to arms 72 and 74 of the coupler 70. The arm 75 of thecoupler 70 is not used.

The couplers 22 and 25 have arms 27 and 29 respectively which areconnected to detectors 82 and 80.

In use, light is launched by the pigtailed laser diode 10 into arm 12 ofcoupler 14 and then branches into arms 16 and 18 of the coupler 14 so asto receive by the couplers 22 and 25. The light from the coupler 22passes through arm 26, connector 30, fibre lead 35 and into fibre 42.The light from the light source 10 therefore follows arrow A shown inFIG. 1 along the length of the fibre 42 to arm 71 of coupler 70 and thenfrom coupler 70 into arms 72 and 74 and then into fibres 44 and 62. Thelight travelling in the direction of arrow A therefore follows twodifferent paths through the fibres 44 and 62 and is then recombined bycoupler 52 into output arm 50. The light then propagates through, fibrelead 37, connector 32, fibre 33, arm 28, coupler 25 and arm 29 todetector 80. When the light recombines at coupler 52 the lighttravelling through fibres 44 and 62 interferes so as to produce aninterference pattern which is detected by the detector 80.

The light which travels from source 10 into arm 18 and then into arm 24of coupler 25 moves in the direction of arrow B through connector 32,arm 50 of coupler 52 and into arms 54 and 56. The light thereforepropagates along the fibres 44 and 62 in the direction of arrows B asshown and into arms 72 and 74 of the coupler 70. The light is recombinedin the coupler 70 and passes through arm 71 into fibre 42 so that thelight propagates along the fibre 42, through fibre lead 35, connector30, fibre 31, coupler 22 and into arm 27 to be detected by detector 82.Once again, when the light travelling in the direction of arrow Brecombines at the coupler 70 the light travelling through cables 62 and44 is able to interfere because the light has traveled through twodifferent path lengths along the fibres 44 and 62 so that the light willinterfere when it coherently recombines. Thus, the detector 82 is alsoable to detect the interference pattern caused by the interference ofthe light which is travelling through the fibres 44 and 62 in thedirection of arrow B.

Thus, according to this embodiment of the invention twocounter-propagating signals pass through the fibres 44 and 62 of thecables 40 and 60. The first counter-propagating signal is the signalwhich travels in the direction of arrow A and the second signal is thesignal which travels in the direction of arrow B.

If one or the other of the cables 40 or 60 is disturbed a change in theproperty of the light travelling through the cable at the position ofdisturbance will be created. For example, the change in property may bea change in phase of the light signal propagating through the respectivefibres. The change in parameter of the light, such as the change inphase of the light signal, will alter the interference pattern causedwhen the light signals recombine either at the coupler 70 or the coupler52 to thereby change the interference pattern which is received by thedetectors 80 and 82. By determining the time difference between thereceipt of the altered interference patterns at the detectors 80 and 82the location of the disturbance of the respective one of the cables 40or 60 can be calculated so that an indication of where an intrusion hastaken place along the length of the cables 40 and 60 can be identified.The counter-propagating technique for enabling the location of adisturbance to the fibres to be determined is disclosed in our aforesaidInternational application PCT/AU99/01028 and also in our Australianprovisional application number PR3169 filed 16 Feb. 2001. The contentsof this provisional application as well as the International applicationare incorporated into this specification.

In order for the detectors 80 and 82 to be able to calculate the timedifference between receipt of the modified counter-propagating signals,that is the change in interference pattern, the detectors 80 and 82should be synchronised. Alternatively, a single detector could beutilised to detect both of the counter-propagating signals so that thesignal detector has a synchronised reference to enable the timedifference to be determined and which can then be used to determine thelength along the cables 40 and 60 at which a disturbance has occurred.

FIG. 2 shows a layout of the preferred embodiment of the invention inwhich the perimeter of an area 100 is to be guarded or monitored forintrusion. In order to install the system a trench 102 is dug about thearea 100 and the cables 40 and 60 are laid in the trench so as to have agenerally zig-zag and overlapping pattern as clearly shown in FIG. 2.This pattern spaces the cables 40 and 60 from one another and alsoensures that a substantial width of detection region is provided. In thepreferred embodiment of the invention the cables 40 and 60 are buried 50mm to 80 mm below the surface of the ground. The trench 102 preferablyhas a width in the direction of double headed arrow W in FIG. 2 ofbetween 1 m and 2 m. When the cables 40 and 60 are buried in the trench102 the cables are obviously invisible to the naked eye and thereforelocation of the perimeter and the existence of the detection system cannot be identified by any person attempting to intrude into the area 100.

Obviously, rather than be of the general u-shape as shown in FIG. 2 thearea 100 can be completely enclosed by the trench 102 and the cables 40and 60, so as to provide a complete monitoring region about the area100.

The preferred embodiment of the invention includes an enclosurecontainer 120 into which the ends of the cables 40 and 60 project. Thecoupler 70 and the exposed fibres which join with the coupler 70 aresealed within the enclosure 120 to prevent ingress of dirt and moisture.The closure 120 can then be buried in the trench 102 with the cables 40and 60.

Similarly, at the other end of the cables 40 and 60 an enclosure 140 isprovided which houses the coupler 52 and the associated exposed fibresso as to prevent ingress of moisture and dirt. Once again, the enclosure140 is buried in the trench with the cables 40 and 60.

A feeder cable 130 preferably also extends into the enclosure,140 andcontains the fibre leads 35 and 37 which join with the connectors 32 and30. Thus the feeder cable 130 can extend to the location of thetransmitting and detecting station 5 so as to couple with the fibres 31and 33.

When the system is installed the trench 102 therefore provides aneffective monitoring perimeter about the area 100. Any person attemptingto gain access into the area 100 will walk over the trench 102 and theweight of the person will apply a load to the cables 40 and or 60 orpossibly move the cables 40 and/or 60 as the person walks over the widthof the trench 102. The load or movement of the cables 40 and 60 will inturn cause a loading or movement of the fibres 62 or 44 which in turnwill cause a change in the aforementioned parameter of thecounter-propagating signals passing through the fibres. This change inparameter, such as a change in phase of the signal, will change theinterference pattern when the phase changed signal recombines with thesignal travelling through the other of the fibres so as to cause achange in the interference pattern.

Detection of the changed interference pattern by one of the detectors 80or 82 provides an indication of an intrusion over the trench 102. Theintrusion can be monitored by mere visual inspection of the interferencepattern or by an alarm signal such as an audible or visual alarm signalbeing generated upon change of interference pattern indicative of anintrusion across the cables 40 and 60. The location of the intrusion canbe determined by the time difference between receipt of the changedinterference pattern at the detector 80 compared with the changedinterference pattern at the detector 82. This enables personnel to bedispatched to the appropriate place to intercept the intruder.

For example, if an intruder attempts to make an intrusion at position X,the intruder will walk over the trench 102 which is not detectable tothe naked eye and merely is just a continuation in the ground fromoutside the area 100 to inside the area 100. The intruder will, forexample, step immediately above or very close to the cable 40 atlocation 40′ for example. This will apply a loading or a movement to thefibre 44 which will change the property of the counter-propagating lightsignals travelling through those fibres. Thus, the modified, or phasechanged signals A and B will propagate from the position 40′ in cable 40in the direction of arrow A and also in the direction of arrow B. Thetime taken for the modified signal to travel from the point 40′ in thedirection of arrow B to coupler 70 where it will cause a changedinterference pattern when it interferes with the signal B travellingthrough the fibre 62, compared with the time taken for the modifiedsignal to travel in the direction of arrow A from the location 40′ tointerfere at coupler 52 with the signal travelling in the direction ofarrow A in fibre 62, will provide an indication of the distance alongthe trench at which the disturbance has occurred. Therefore appropriatepersonal can be dispatched to the region of the disturbance to interceptthe intruder.

If multiple wavelength sources are utilised, preferably the couplers 14,22, 25, 52 and 70 are wavelength multiplexing/de-multiplexing waveguidecouplers to thereby minimise loss of signal when the signals arecombined or separated by the couplers.

If the length of the cables 60 and 40 are particularly long the fibres35, 37, 42, 44 and 62 may include optical amplifiers along their length.Because the fibres convey signals in both directions in order to providethe counter-propagating signals discussed above, it is necessary thatany amplifier station accommodate the travel of the signals in bothdirections along the fibres. Thus, if the optical amplifiers are notbi-directional, an amplifier assembly of the type disclosed in ouraforesaid provisional application filed 16 Feb. 2001 can be utilised.

The preferred embodiment of the invention has the advantage that theburied cables 40 and 60 are sensitive enough to detect even theslightest foot-fall, continuously and discretely, twenty-four hours aday everyday for many years. Their performance is completely unaffectedby changes in the local environment (rain, hail, temperature, electricalstorms and magnetic loads). Noise and vibration effects from backgroundtraffic can be screened out. Washouts do not disable the system and canbe repaired.

The system also has the advantage that it is non-detectable in that thefibres cannot be detected by metal detectors because no metal isrequired in the cables, the fibres can also not be detected by emissionsbecause there is no electromagnetic radiation emanating from the fibresand, assuming that the region of the trench 102 is restored to itsoriginal condition before digging, the location of the cables and 40 and60 are impossible to detect. The sensitivity of the detecting system andtherefore the provision of any alarm condition, can be set or changed atwill to suit the local environment and the operators needs. Cablesensitivities aren't effected by is lengths up to 60 km and cabling caneasily be extended to 350 km or more (using appropriate amplification ifdesired). Hence, trench lengths of up to 70 km are possible or areas ofgreater than 125,000 m².

Extensive systems may be broken into multiple zones, each of the whichmay have different sensitivity levels set. Sensitivity levels may bepreset at different values for different time zones of the day.

For maximum effect the cables should be laid in a shallow trench atleast 1.8 m wide, for the entire length of the sensitive zone. This areamust be excavated by a suitable machine or by hand to a uniform depth ofbetween 50 and 80 mm. The soil removed in the process is used tobackfill the trench once the cables are laid. The trench base need notbe flat and no particular care is needed to maintain a particular depthor uniformity.

The cables are terminated at each end of the trench or at 35 only oneend. Provision must be made to connect the sensor cables to a feedercable 130 at one end of the trench. The feeder cables(s) is contained insuitable PVC conduit, from the trench to the position of the computerterminal. This conduit should be buried at least 200 mm below thesurface of the ground until it can penetrate a wall or floor of abuilding or cabinet. The sensor cables should be normal, commercialgrade 2 core or 4 core tight buffered optical fibre communicationcables, usually 6 mm in diameter. Preferably two cables are required foreach system. They are preferably identical.

The sensor cables are laid along the bottom of the trench, in a closelyspaced wave or zig-zag pattern that runs across the full width of thetrench. It is essential that the wave or zig-zag pattern of one cable isopposite (a mirror image) to that of the other cable, ie they are 180%out of phase, see FIG. 2. The cables may touch as they cross over. Thereis no need to maintain close control over the relative depths of the twocables.

For maximum sensitivity the spacing between the two opposing wave peaksshould be in the 400-500 mm range. A wider spacing may still beeffective, but the sensitivity beings to fall off if the spacing exceeds500 mm.

Once the cables are laid, spliced to the feeder cables(s) and tested,they may be buried. The cables should not be lifted or substantiallymoved during the back filling operation and hence it is recommended thatthe first 40-50 mm of fill should be done by hand or more carefully witha small machine. This should then be roughly leveled and consolidated bya light roller or a tamping machine. The remainder of the soil can thenbe backfilled and consolidated with normal earth moving plant such as afront-end loader. The surface should be smoothed, with an allowance forslumping, and then re-grassed if appropriate.

Although arms 15, 17, 19, 53 and 75 of the various couplers are not usedin the embodiment described above, those arms could be used forpower/maintenance monitoring.

Furthermore, although the preferred embodiment has been described withreference to the counter-propagating signals which traverse through thefibres 44 and 62, the fibres 44 and 62 could merely include a signalwhich traverses in only one direction and in this embodiment the fibres44 and 62 are not joined but rather have ends which are polished to formmirrors so that the light signal is reflected back through the fibres 44and 62 to the coupler 52 where those signals interfere to produce theinterference pattern. This embodiment provides sensitivity and willalert to an intrusion but will not enable the location of the intrusionto be identified.

Since modifications within the spirit and scope of the invention mayreadily be effected by persons skilled within the art, it is to beunderstood that this invention is not limited to the particularembodiment described by way of example hereinabove.

1. A perimeter security system including; at least a first waveguide andat least a second waveguide buried below ground level and extendingalong a perimeter which defines an area to be monitored; means forlaunching light into the first and second waveguides; a detector fordetecting light which has propagated through the waveguides so as todetect a change in a parameter of the light propagating through thewaveguides due to an intrusion across the ground beneath which thewaveguides are buried and for providing an indication of that intrusion;the first and second waveguides being provided in separate cables andthe separate cables being buried beneath ground level in zig-zag spacedapart relationship with respect to one another to define a perimeterregion having a substantial width which will be traversed by a personintruding into the area; a first of the said cables contains said atleast one waveguide and a second said cable contains said secondwaveguide; a further waveguide being contained within the first cable;first coupling means at one end of the said first, second and furtherwaveguides for coupling the waveguides so that light launched into thesaid further waveguide is able to propagate through the furtherwaveguide and then into the said first and said second waveguides topropagate in a first direction through the said first and secondwaveguides; second coupling means at the other end of said first andsaid second waveguides so that the light propagating in the said firstdirection through said first and second waveguides is able to coherentlyrecombine and interfere at the second coupling means; and light alsobeing able to be launched through said second coupling means and intosaid first and second waveguides to travel in a direction opposite saidfirst direction and coherently recombine at the first coupling means sothe light travelling in the opposite direction is able to interfere andthen propagate through the said further waveguide.
 2. The perimetersecurity system of claim 1 wherein the detector detects the interferencepattern and upon an intrusion a parameter of light passing through oneof the waveguides is altered with respect to the same parameter of thelight passing through the other of the waveguides, to thereby change theinterference pattern detected by the detector to provide an indicationof the intrusion.
 3. The perimeter security system of claim 1 whereinthe substantial width is a width such that a person travelling in normalwalking or running motion will not step over the width of the region. 4.The perimeter security system of claim 3 wherein the width of the regionis between one and two meters.
 5. The perimeter security system of claim1 wherein counter-propagating light signals are launched into each ofthe waveguides so that the location of an intrusion can be detected bythe time difference between detection of the changed interferencepattern propagating in one direction and to the changed interferencepattern propagating in the opposite direction.
 6. The perimeter securitysystem of claim 1 wherein the detector is coupled to the furtherwaveguide and to the second coupling means for detecting the counterpropagating light signals after interference of those signals so thatany disturbance of the first waveguide and/or said second waveguide willchange a parameter of the light propagating through the first and/orsecond waveguides to thereby change the interference patterns detectedby the detector to cause the detector to provide an indication of theintrusion.
 7. The perimeter security system of claim 6 wherein thelocation of the intrusion can be determined by the time differencebetween receipt of the modified counter-propagating signal travelling inthe first direction compared to the receipt of the modified propagatingsignal travelling in the opposite direction.
 8. The perimeter securitysystem of claim 6 wherein the detector comprises a first detector and asecond detector, the first detector and second detector beingsynchronised and the first detector detecting the counter-propagatingsignal travelling in the first direction and the second detectordetecting the counter-propagating signal travelling in the oppositedirection.
 9. The perimeter security system of claim 1 wherein the meansfor launching light into the waveguides comprises a light source coupledto a third coupling means having first and second output arms, the firstoutput arm being coupled to an input arm of a fourth coupling means andthe other output arm being coupled to an arm of a fifth coupling means,an arm of the fourth coupling means being coupled to the furtherwaveguide for launching light into the further waveguide, and an arm ofthe fifth coupling means being coupled to an arm of the second couplingmeans for launching light into the second coupling means.
 10. Theperimeter security system of claim 8 wherein the first detector iscoupled to an output arm of the fourth coupling means and the seconddetector is connected to an output arm of the fifth coupling means. 11.A perimeter security system for underground use including: at least afirst waveguide and at least a second waveguide for extending along aperimeter which defines an area to be monitored; means for launchinglight into the first and second waveguides; a detector for detectinglight which has propagated through the waveguides so as to detect achange in parameter of the light propagating through the waveguides dueto an intrusion across the ground when the waveguides are buried, andfor providing an indication of that intrusion; the first and secondwaveguides being provided in separate cables, and the separate cablesbeing for location beneath ground level in a zig-zag spaced apartrelationship with respect to one another to define a perimeter regionhaving a substantial width which will be traversed by a person intrudinginto the area; a first of said cables containing said at least onewaveguide and a second said cable containing said second waveguide; afurther waveguide being contained within the first cable; first couplingmeans at one end of said first, second and further waveguides forcoupling the waveguides so that light launched into said furtherwaveguide is able to propagate through the further waveguide, and theninto the said first and said second waveguides to propagate in a firstdirection through the first and second waveguides; second coupling meansat the other end of said first and said second waveguides so that thelight propagating in said first direction through said first and secondwaveguides is able to coherently recombine and interfere at the secondcoupling means; and light also being able to be launched through saidsecond coupling means and into said first and second waveguides totravel in a direction opposite said first direction, and coherentlyrecombine at the first coupling means so that the light travelling inthe opposite direction is able to interfere and then propagate throughsaid further waveguide.
 12. A below ground perimeter security systemincluding: a first cable containing at least one first waveguide; asecond cable containing at least one second waveguide; the first andsecond cables being arranged below ground level and in spaced apartrelationship relative to one another to define a barrier region which,should the region be traversed at ground level, will result in detectionof the traversing of that barrier region; means for launching light intothe first and second waveguides, so that the light is able to eithercirculate through the first and second waveguides in counter propagatingmanner, or be reflected from respective ends of the first and secondwaveguides and propagate back along the respective first and secondwaveguides into which the light was launched; means for receiving thelight from the first and second waveguides so that the light caninterfere; and a detector for detecting the interfering light from thefirst hand second waveguides to detect a change in a parameter of thelight propagating through the first and second waveguides due to thetraversing of the barrier region to provide an indication of anintrusion across the barrier region.
 13. The system of claim 12, whereinthe first and second waveguides are coupled together by a coupler sothat the light circulates through the waveguides in counter propagatingmanner to enable not only the detection of intrusion, but also thelocation of the intrusion.
 14. The system according to claim 12, whereinthe first and second waveguides are each provided with a reflective end,and light is reflected from the reflective end back along the first andsecond waveguides.